FIG. 1 shows a conventional gamma ray spectroscopy system 10. A source of high voltage 12 provides power to a detector 14. A gamma ray 16 emitted from a source of nuclear decay is converted by the detector 14 into an electrical analog pulse signal in line 18. The analog pulse signal is typically amplified in a preamplifier 20, shaped in a shaping amplifier 22, and stretched in a pulse stretcher 24. After the analog pulse signal is amplified, shaped, and stretched, it is converted into a digital signal by an analog to digital converter 26. The analog to digital converter (ADC) outputs an n-bit distal signal (e.g., a 12-bit digital signal) in line 27. The digital signal in line 27, which in gamma ray spectroscopy represents the energy of the detected gamma ray, is then counted in a binning scheme to produce a histogram 30, i.e. an energy spectrum of the incoming gamma rays.
A hardware implemented binning scheme is shown in FIG. 1, although binning schemes can also be implemented in software or firmware. In FIG. 1, the N-bit digital signals in line 27 input to an add-one circuit 28. The add-one circuit 28 outputs a digital binning signal in line 29 that includes a memory address, and instructions to increment the data in a memory channel at the address by a count of one. The digital binning signal in line 29 from the add-one circuit 28 inputs random access memory (RAM) 31, and consequently increments data in the appropriate memory channel. The raw data in the RAM 31 can be displayed as a histogram 30 on a display screen.
In an ideal spectroscopy system, an exact linear relationship would exist between the input radiation 16 and the measured histogram 30. All events of a particular type (e.g., emitted gamma rays having a specific energy) would fall in the same bin or channel of the histogram 30. Such an ideal histogram might look like the histogram shown in FIG. 2.
Conventional ADCs 26 convert analog pulse signals into digital pulse signals by sorting the analog pulse signals into one of a number of contiguous channels. The particular channel (i.e. the particular value of the digital pulse signal) depends upon the value of the analog pulse signal, but also on the differential linearity or width of the channels in the ADC 26. It is generally believed that the channels in the ADC should have equal or near equal width for nuclear spectroscopy to be accurate enough for useful analysis.
In order to achieve a near ideal histogram 30, a variety of ADC techniques have been developed to accurately and quickly convert analog pulses to digital values. Spectroscopy grade ADCs have been designed to improve the differential linearity of the ADC (i.e. ensure that the channels have equal width across the spectrum). However, even with an expensive spectroscopy grade ADC having very good differential linearity, the histogram will not be accurate if non-linearities are present in the detector or elsewhere.
Detectors used for detecting gamma rays in nuclear spectroscopy systems include: Geiger-Muller tubes, sodium iodide scintillation detectors, plastic scintillators, silicon (lithium) detectors, gas flow proportional counters, germanium (lithium) detectors and hyper-pure germanium detectors. Geiger-Muller tubes are very inexpensive but have essentially no energy resolution. That is, an analog pulse signal from a Geiger-Muller tube does not differentiate between incoming gamma rays according to energy. In contrast, hyper-pure germanium detectors have excellent resolution and are extremely linear in terms of energy over a wide variety of energies. However, hyper-pure germanium detectors can cost tens of thousands of dollars, require liquid nitrogen for cryogenic cooling, and are physically large.
Sodium iodide scintillation detectors, and many other scintillation-type detectors, have reasonable energy resolution, are rugged, do not require cryogenic cooling, are physically small and have a reasonably low cost. Sodium iodide detectors are therefore desirable for use in many applications in medicine, radiation surveying, waste monitoring, and education. Unfortunately, sodium iodide detectors suffer from a variety of problems:
1) The pulse height of the analog pulse signal from a sodium iodide detector is not normally proportional to the energy of the incoming gamma ray below approximately 200 keV (i.e. integral non-linearity), PA1 2) A mono-energetic gamma ray source will produce a peak with substantial width (i.e., a sodium iodide detector has only fair energy resolution), and PA1 3) The resolution of the sodium iodide detector is a significant function of energy (i.e., low energy peaks are much narrower than high energy peaks).
Despite these problems, much effort has gone into sodium iodide spectroscopy. In the low energy region of the spectrum, peaks are normally 5 to 7 keV in Full Width at Half Maximum (FWHM), so each channel of the histogram needs to represent 0.5 to 2.0 keV to be acceptable for analysis. With conventional spectroscopy ADCs in which each channel has the same width, an ADC needs at least 1000 to 4000 channels to cover a spectrum from 0 to 2000 keV. In such a sodium iodide system, high energy peaks have a resolution of 100 to 120 keV FWHM. Such a peak would cover 50 to 200 channels in a system with 1000 to 4000 channels. If weak signals are present, it may be difficult to locate peaks at high energies. Some have used lower resolution ADCs to overcome the problem with weak peaks at high energies, but this sacrifices sufficient resolution to analyze low energy data.
Historically, the focus on sodium iodide spectroscopy has been on producing electronics that are more precise and more linear, so that the spectrum can be captured in detail, and the results can be unraveled by sophisticated analysis, usually in a laboratory. Also, as mentioned above, new detector technologies with better linearity and better resolution over the entire range of interest have been developed, but these systems are bulky, expensive, and difficult to operate.
It is therefore desirable to provide a practical system of spectroscopy that allows the use of sodium iodide detectors, or other similar detectors, without sacrificing system performance.